The reaction of reduction of binuclear ruthenium(IV) oxochloride complex with lithium coutercations Li4Ru2OCl10 in HCl solutions is studied by electrochemical and spectrophotometric methods. The results are consistent with the process involving the irreversible reduction of the dimer in weakly acidic medium (0.2–0.8 М HCl) to form a mixed-valence intermediate which decomposes later to Ru(IV) and Ru(III) monomers. Increasing the HCl concentration to 2.0 М and higher excludes its formation. The kinetics of the reduction of Ru(IV)–Ru(IV) dimer by coordinated water in 3 М HCl obeys the bimolecular law. Binuclear complexes self-assemble into the tetranuclear catalyst which oxidizes water to oxygen molecule by the four-electron mechanism. The composition of the tetranuclear catalyst is determined by means of electrospray mass spectrometry.